546 
SHARKS AND RAYS. 
by the shortness of the thin tail, which always bears a serrated spine, and may 
have a rudimental fin; the minute teeth being either singly or triply cuspidate. 
The oldest representative of the family seems to be the extinct Cyclobcdis from 
the Cretaceous rocks of Palestine, in which the disc is either circular or oval in 
form, the tail very short, only slightly projecting beyond the margin of the disc, 
and devoid of either spine or fin, while the upper surface of the body has one or 
more longitudinal series of large spiny tubercles running backwards from the 
pectoral girdle, the remainder of the body and disc being more or less sparsely 
covered with minute prickles. 
The Extinct Lobe-Finned and Fold-Finned Sharks,— Orders Ichthyotomi 
and Cladodontia. 
The whole of the preceding members of the subclass are included in a single 
order, the characters of which have been already described; but in the Palaeozoic 
strata of both Europe and the United States there occur remains of extinct sharks, 
indicating two perfectly distinct ordinal groups. 
Lobe-Finned The essential characteristic of this group, as shown in the restored 
Group. skeleton figured on p. 317, is the lobed structure of the pectoral fins, 
which consist internally of a long tapering segmented axis, from which are given 
off a double series of cartilaginous rays, 
as shown in the figure on p. 319. The 
internal skeleton of these sharks shows 
granular calcifications in the cartilage; 
but the notochord is never or but seldom 
constricted into distinct vertebrae, the 
calcification, except in the tail, stopping 
short at an incomplete stage, when the 
bodv of each segment of the backbone 
consists of three separate pieces, as in 
the example figured on p. 312. The 
upper and lower arches and spines of the 
backbone are tall and slender; the upper spines having no intercalary cartilages 
between them. As represented by the genus Fleur acanthus, common to the 
Permian and Carboniferous rocks of both sides of the Atlantic, these sharks are 
further characterised by the slender and slightly depressed form of the body, the 
terminal position of the mouth, and the dipliycercal tail. The long and low 
dorsal fin is continued along the whole of the back from a short distance behind 
the head, and its cartilages are more numerous than the subjacent spines of the 
vertebrae; immediately behind the head is a long barbed spine, and the body was 
probably devoid of shagreen. The teeth, as shown in the annexed illustration, 
are very peculiar, consisting of two divergent and generall}' unequal-sized cones, 
supported on an expanded base. 
Fold-Finned The oldest and most primitive representatives of the entire 
Group. subclass are the armoured sharks of the Devonian and lower 
Carboniferous epochs, especially characterised by the simple structure of their fins, 
TEETH OF A LOBE-FINNED SHARK.— After Fritsctl. 
